A tundish is a large intermediate holding vessel for molten metal used in continuous casting processes, such as the continuous casting of steel. The tundish is, in effect, an intermediate process reservoir which receives a large quantity of molten metal from conveying ladles exiting a furnace in which actual smelting of the ores or refining of molten metal occurs, and which then transfers the molten metal to a casting system. A system of inlet and outlet nozzles controls the flow of molten metal into and out of the tundish.
The tundish itself is generally a steel vessel which is lined with several layers of a refractory composition. A permanent lining, generally of refractory brick, serves as an inner lining to protect the vessel. The permanent lining, in turn, is coated with a wearable and disposable lining, generally of a refractory composition which has been applied to the permanent lining by gunning, spraying, trowelling, or dry vibration. The disposable lining is in direct contact with the molten metal in the tundish and protects the permanent lining from exposure to the molten metal.
In general, there are two types of monolithic refractory compositions which are used as disposable tundish linings, those which require the addition of a liquid phase, usually water, in order to apply the composition to the permanent lining and those which do not require a liquid phase for the application of the composition to the permanent lining. Those compositions which require a liquid are applied by either gunning, spraying, or trowelling. Those compositions which do not require a liquid are called "dry vibratables".
A dry vibratable is generallly installed by one of two methods, one which uses a former to control the application thickness and the other which does not require the use of a former. The former is shaped to the contour of the inner dimensions of the vessel. This method allows for the even application of a desired thickness of monolithic refractory (dry vibratable) in large or complex vessels. The method for application which does not require a former, namely, the so called "no-former" method, involves preheating the vessel from about 1200.degree. F. to about 2000.degree. F. once preheated, the vessel is completely filled with the dry monolithic refractory. The refractory is allowed to cure in the vessel for about 1-5 minutes. After this time has elapsed, a working lining of finite thickness is formed and the uncured refractory is reclaimed for reuse. This method allows the control of application thickness by time allowed for curing. This method is useful for lining small or uncomplicated vessels.
In the continuous casting process of steelmaking, there are two general tundish practices, cold practice and hot practice. Cold practice means that the tundish is at ambient temperature when steel is conveyed from the ladle into the tundish. Hot practice involves preheating the tundish to temperatures ranging from about 1200.degree. F. to about 2500.degree. F. Thus, the steel is conveyed from the ladle to a "hot" tundish. The use of hot practice in steelmaking has resulted in higher quality steel being produced from the continuous casting process. The higher quality from the hot practice has been related to, among other things, reduced chilling effects, reduced gas evolution and reduced turbulence in the preheated tundish due to reduced gas evolution from the preheated refractory lining.
Traditionally, only the gunnable, sprayable, or trowellable monolithic refractories have been used as disposable linings for hot practice tundishes. Dry vibratable refractories were not used in this application because their bonding system was not sufficient at preheat temperatures. The traditional dry vibratable bond system is comprised of two parts, a low temperature bonding agent, is usually an organic resin, and a high temperature bond system, usually a silicate. In order to facilitate preheating, an intermediate temperature bonding agent was needed. This has been accomplished in the present invention by replacing the silicate with either a phosphate or silicate/metal powder bonding system which gives the refractory the necessary strength at both intermediate and high temperatures. The strength at these temperatures is necessary to ensure that the dry vibratable monolithic refractory remains in place to act as the tundish working lining.